Galaxy MMS sending and receiving issue

We recently switched from AT&T to Xfinity Mobile and in the process purchased the two Samsung Galaxy A50's from Xfinity for my wife and myself. We activated both phones recently and initially everything functioned properly. On Sunday, 2/9, they both received a 426 Mb firmware update. Since that update neither phone can send nor receive MMS messages unless I disable WiFi. Both phones received another 126 Mb firmware update on 2/19 which unfortunately did not resolve the issue- it appears to have been the February security update. Both A50’s are currently on version A505USQS4ATA3. Our son’s unlocked Galaxy S8 had performed flawlessly on the AT&T network since we purchased it, however as soon as we inserted the Xfinity SIM card and activated the Xfinity Mobile service he encountered the identical MMS issues our A50’s are experiencing. I have tried two third-party messaging apps- both with the same result. I have tried it with Wi-Fi calling on and off- it makes no difference. I have checked the APN settings and they appear to be correct. I have reset the network settings, cleared the device cache, and finally performed a full factory reset- all to no avail. I have contacted both Samsung and Xfinity support, also to no avail- each blames the other. This is extremely inconvenient because both my wife and I have elderly parents we are managing and communicating via group text messages is critical to us. I am hoping that perhaps you can help me / us resolve this issue. Thank you for your time- I appreciate it very much!